1. Field of the Invention
The present invention relates to an image forming apparatus such as a color copying machine, a color printer or the like, and particularly, to an image forming apparatus which forms images on a plurality of image carriers, and sequentially transfers these images onto a transfer material such as a paper sheet, to obtain a hard copy.
2. Description of the Related Art
Conventionally, in many color image forming apparatuses of an electrophotography method, yellow, magenta, cyan, and black toner images are sequentially formed for every one turn of a photoconductive drum as an image carrier, and the toner images are sequentially transferred to a paper sheet. In this method, since a photoconductive drum must rotate for four turns to form one color image, there is a problem that the image forming speed is low.
Therefore, in recent years, a proposal has been made as to an image forming apparatus of a four continuous tandem method in which four photoconductive drums are disposed so that the image forming speed is increased. In this method, four photoconductive drums are arranged in parallel with each other, on which yellow, magenta, cyan, and black toner images are formed, respectively, and these toner images are sequentially transferred to one sheet of transfer material retained and fed by a transfer material feed belt, to obtain a color image. This photoconductive drum four continuous tandem method has an advantage in that the image forming speed is four times higher than the method as described above.
However, in the image forming apparatus having the four photoconductive drums, a total of four transfer corona chargers for electrostatically transferring toner images formed on photoconductive drums must be respectively provided so as to correspond to the photoconductive drums. In addition, this image forming apparatus requires a suction corona charger for electrically suctioning a transfer material to a transfer material feed belt, and an AC corona discharger or the like for discharging electronic charges remaining on the feed belt, so that these remaining charges do not prevent suctioning effects of the suction corona charger. This method thus uses a greater number of corona discharging generators than the other conventional method described in the beginning, unavoidably resulting in increases in generation of the amount of ozone. Therefore, a large-scale ozone remover apparatus or the like must be installed additionally to cope with the ozone, which leads to a problem in view of costs and down-sizing of the apparatus.
Further, since the photoconductive drum four continuous tandem method reproduces an image of predetermined colors by sequentially feeding a transfer material through four transfer positions thereby overlapping four toner images, a dislocation may be incurred between toner images to be transferred and thus seriously deteriorates the image quality, if the transfer material slips or slides during feeding. In the case of a full-color printer, even a slight dislocation between overlapping positions of respective toner images makes reproduced colors absolutely different from desired colors. Therefore, it is necessary to sufficiently gain a high suction force for suctioning a transfer material against the transfer material feed belt to eliminate dislocations of the transfer material. However, the suction force cannot be increased too high since a mere increase in suction force rather affects transferring of toner images. For these reasons, there is a problem that a color dislocation easily occurs due to an insufficient suction of a transfer material against a transfer material feed member.